Overpopulation is a phenomenon in which a species population becomes larger than the carrying capacity of its environment. It is a state whereby the human population rises to an extent exceeding the carrying capacity of the ecological setting. Overpopulation can be caused by increased birth rates, lowered mortality rates, medical breakthroughs, and increased immigration. Overpopulation can cause environmental deterioration, worsening in the quality of life, or even the disintegration of the population.

In ecology, overpopulation is a concept used primarily in wildlife management. Typically, an overpopulation causes the entire population of the species in question to become weaker, as no single individual is able to find enough food or shelter. As such, overpopulation is thus characterized by an increase in the diseases and parasite-load which live upon the species in question, as the entire population is weaker. Other characteristics of overpopulation are lower fecundity, adverse effects on the environment (soil, vegetation or fauna), and lower average body weights.

Judgments regarding overpopulation always involve both facts and values. Animals are often judged overpopulated when their numbers cause impacts that people find dangerous, damaging, expensive, or otherwise harmful. Societies may be judged overpopulated when their human numbers cause impacts that degrade ecosystem services, decrease human health and well-being, or crowd other species out of existence.

Population density is not an adequate measure of overpopulation, since countries with advanced economies, like the Netherlands or Hong Kong, can support an extremely dense population. Whether a country is overpopulated or not depends on its population growth rate, standard of living, lifestyle, culture, available technology and resources, economy, and other factors.

Overpopulation can result in an increased demand for food, water, housing, energy, healthcare, transportation, and more, which contributes to ecological degradation, increased conflicts, and a higher risk of large-scale disasters like pandemics. The United Nations is taking action to manage the dangerous effects of overpopulation without diminishing the quality of life.